STRUCTURAL AND STRATIGRAPHIC DEVELOPMENT OF THE TOCO REGION OF
THE NORTHERN RANGES, N.E. TRINIDAD, AND IMPLICATIONS FOR THE TECTONIC DEVELOPMENT OF THE SOUTHEASTERN CARIBBEAN
ST. ALGAR and J.L. PINDELL,
Department of Earth Sciences, Dartmouth College, Hanover, NH 03755, U.S.A.
Detailed mapping of the Toco region of the eastern Northern ranges in northeastern Trinidad has led to new interpretations
of the lithostratigraphy, sedimentology, and structure of the area, which help to refine our understanding of the
likely tectonic evolution of the southeastern Caribbean.
Previous work in the Toco region consisted of lithologic descriptions and mapping (Barr 1963). In our interpretation,
the Grande Riviere, Rio Seco and Tompire Formations have been tentatively replaced by the Rampanalgas and Guayamara
Formations. The Galera and Toco Formations are still recognized, though their boundary has been realised to be
a right-lateral shear zone and not an unconformity as had been thought previously. Recognition of the shear zone
leads to the interpretation that the Toco Formation is allochthonous with respect to the other formations, though
the magnitude of allochthoneity is uncertain. Thus, the new stratigraphic order for the possibly conformable Cretaceous
formations is, from oldest to youngest, Rampanalgas, Guayamara, Galera. This sequence is thought to represent a
northward prograding submarine fan sequence, sourced from South American quartzose, cratonic areas.
Three phases of deformation have been identified in the Toco region: an early compressive phase, forming both large
and small scale folds with 0450/2250 axes; a second phase of transpressive, right lateral wrenching producing slightly
compressive right lateral strike-slip faults, striking 0450/2250, along with left lateral strike-slip faults striking
broadly southeast; and a final phase of reverse faulting which reactivated the existing strike-slip faults as well
as forming new fault planes striking 0750/2550. These same phases of deformation have been recorded in the western
and central portions of the Northern Ranges by the authors, but their trends are oriented 450 clockwise of those
in the Toco region. We suggest that this is because, like the rest of the Northern Ranges, and indeed much of the
southeastern Caribbean, the Toco region is composed of rocks which have been deformed by the eastward passage of
the southeastern corner of the Caribbean Plate with respect to the South American Plate, but unlike the western
and central Northern Range rocks they have yet to be rotated into the E-W alignment which had been produced by
continued eastward movement of the Caribbean Plate. This means that the Northern Ranges have only recently entered
the southern Caribbean Plate Boundary Zone, and have therefore probably not been transported eastward from their
site of deposition by any large amount. Thus, though some strike-slip has occurred along the El- Pilar fault, the
displacement is on the order of tens of kilometers rather than hundreds, the major motion between the Caribbean
and South American Plates in this area, having occurred to the north of the Northern ranges, along the Northern
Boundary Fault.
As the primary phase of deformation in the Northern Ranges affected an already lithified Upper Cretaceous Galera
Formation with an intensity apparently equal to that of older formations, and since the oldest known unmetamorphosed
rocks overlying the deformed Northern Range rocks are of Miocene age, the major orogeny affecting the Northern
Ranges must have occurred sometime between the late Cretaceous and the late Miocene, and not in the medial Cretaceous
as had previously been suggested.
Unlike previous interpretations of the Northern Ranges, all of the above ideas are consistent with regional kinematics
and tectonic models for the Caribbean (Pindell et. al., 1988, Robertson and Burke, 1989, Pindell and Barrett, 1990),
as well as the 40k-
3OAr cooling age of 20-30 Ma, obtained by Speed and Foland (1989) on micas from the western and central portions
of the Northern Ranges.
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